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Related to Window stool: window apron
In a window trim, the nosing directly above the apron; the horizontal member of the window finish which forms a stool for the side casings and conceals the window sill.
window stool, window board, elbow board
A horizontal board on a windowsill, fitted against the bottom rail of the lower sash and between the sash frame stiles; forms a base on which the casing rests; usually of wood, but may be of metal or other facing material.